This application claims the priority of German Patent Applications, Serial No. 199 22 542.7, filed May 10, 1999, and Serial No. 100 23 488.7, filed May 9, 2000, the subject matter of which is incorporated herein by reference.
The present invention relates to a process for producing welded steel pipes with a high degree of strength, ductility and deformability, in particular line pipes, using the UOE-process. More particularly, the invention relates to a heat post-treatment of the popes after the welding and sizing operation.
The yield strength of sheet metal employed in the manufacture of pipes by cold molding, for example by the UOE-process, has to exceed at least a minimum specified value, so as to reliably and safely prevent flow of the finished pipe.
Pipes made of high-strength steel with a yield strength Rt0,5xe2x89xa7550 MPa (X80 according to API-5L) can meet these requirements in practice only by having a comparatively high initial upper yield strength ratio due to the viscosity and deformation characteristics that have to be met at the same time. Current industry standards require a maximum upper yield strength ratio of, for example, 0.93 according to API5L, which due to work hardening during molding and sizing of the pipes is difficult to achieve in series production, requires complex manufacturing technology and increases production cost. Moreover, the cold-forming process reduces the integral deformation reserve due to the required high initial yield strength ratio for higher grade steel. Hence, it is difficult to realize in practice the integral deformation reserve xcex5upxe2x89xa72% required of the component when taking into account the typically observed statistical variations on pipes made of steel with a yield strength Rt0,5xe2x89xa7550 MPa (X80). An integral deformation reserve of xe2x96xa1upxe2x89xa72% has so far never been realized on pipes made of steel with a yield strength Rt0,5xe2x89xa7620 MPa (X90). xe2x80x9cIntegral deformation reserve xcex5upxe2x80x9d refers to the average peripheral plastic expansion of the pipe before wall necking, analogous to the elongation before reduction of area in a laboratory tensile tests. (Hohl, G. A. and Vogt, G. H.: Allowable strains for high strength line pipe, 3R international, 31 Yr., Vol. 12/92, p. 696-700).
To remedy this problem, it has been proposed in the past to change the composition of the alloy and/or the rolling technique to achieve the required higher deformation characteristic values. However, the options are limited in practice: on one hand, adding additional alloy materials, such as nickel, make the product significantly more expensive, while adding other alloy materials, such as boron, creates forming problems. On the other hand, the available temperature window, the cooling speed and the strain in the thermal-mechanical rolling process can only be changed within certain limits imposed by the employed technology.
A process referred to as xe2x80x9cbake hardeningxe2x80x9d for increasing the strength of components is known from DE 196 10 675 C1. This process refers to an artificial aging process associated with enamel baking. The component is preferably coated in a zinc bath through which the previously cold-rolled tape passes. The zinc bath temperatures are in a range between 450-470xc2x0 C. To enable reliable surface processing of conventional DP (dual-phase) steels, German Pat. No. DE 196 10 675 C1 discloses a steel with the following composition in wt. %:
0.05 to 0.3% carbon
0.8 to 3.0% manganese
0.4 to 2.5% aluminum
to 0.2% silicon.
The remainder is iron with steel-making related impurities. Cold rolling is followed by a heat treatment, preferably in a hot-dip galvanizing apparatus or in a continuous annealing furnace.
The micro-structure is comprised of a ferritic matrix in which martensite is incorporated in form of islands. The minimum characteristic values attainable by the conventional process are as follows:
The essential elements favored in the process disclosed in German Pat. No. DE 196 10 675 C1 are aluminum and silicon. The element silicon is maintained at a low concentration in order to suppress the formation of red scale during hot-rolling. Red scale poses the danger of drawing in scale that causes surface inhomogeneities when the tape is pickled. A high aluminum fraction promotes formation of ferrite during annealing between the conversion temperatures Ac1, and Ac3. Addition of aluminum also improves the adhesion characteristic of zinc as well as of the zinc-iron alloy layers. The formation of pearlite is moved to significantly longer times and can therefore be suppressed with the achievable cooling rates.
The conventional process cannot be applied to welded pipes made of high-strength steel, for instance grade X80 steel with a minimum yield strength of 550 MPa, since heat treatment in the temperature range of 450-470xc2x0 C. is uneconomical due to the long heating and holding times. High-strength steels such as grade X65 steel, have a ratio of yield strength to tensile stress of  greater than 0.70, other steels have a ratio in the range between the 0.80-0.93.
It would therefore be desirable to develop a process for manufacturing welded steel pipes with a high degree of strength, ductility and deformability, in particular line pipes, using the UOE-process, wherein the process can be used to produce economically and reliably steel with grades xe2x89xa7X80 with a minimum yield ratio of 550 MPa as well as acid gas-resistant grades, while maintaining the upper limit of the ratio of yield strength to tensile stress set by current industry standards.
The invention is directed to a process for producing welded steel pipes with a high degree of strength, ductility and deformability. In particular, the invention incorporates a heat post-treatment after the welding and sizing operation.
According to one aspect of the invention, a steel sheet with a composition (in wt. %) of 0.02 to 0.20% carbon; 0.05 to 0.50% silicon; 0.50 to 2.50% manganese; and 0.003 to 0.06% aluminum, the remainder representing iron with steel-making related impurities, is cold-formed into a pipe shape, welded and sized. The so obtained pipe undergoes heat post-treatment in a temperature range of 100-300xc2x0 C. wherein the holding time is adapted to the pipe wall thickness. The pipe is subsequently cooled in air or by forced cooling. The holding time depends primarily on the wall thickness of the heated component and to a lesser extent on the type of heat supply. The pipe produced in this manner has the same high mechanical strength as conventionally produced pipes, but has more than twice the deformation reserves, without exceeding the upper limit for the ratio of yield strength to tensile stress set by current industry standards.
Advantageous embodiments may include one or several of the following features. The heat treatment can be performed in a continuous annealing furnace or by passage through an induction coil and/or induction furnace. In addition, the heat treatment can be performed in conjunction with the application of an outside insulation layer which can be a mono-layer or a multi-layer structure. The holding time can vary in extreme cases between seconds and several hours.
The pipes can be welded with a helical seam or a straight seam. Pipes having a straight seam can be presized before the heat treatment by a combined application of cold-expansion and cold-reduction, wherein the order and the degree of expansion and reduction is determined by the requested pipe profile.
Optimal results are achieved when the minimum initial yield strength of the sheet metal matches the minimum yield strength of the pipe after subtracting the increase of the yield strength due to cold-forming and heat treatment effects. A pipe fabricated in this way is resistant to aging and has particularly homogeneous properties along the periphery of the pipe.
According to another embodiment of the invention, additional elements can optionally be added to the alloys up to the previously described upper limits. For example, up to 0.02% phosphorus; up to 0.06% titanium; up to 0.20% chromium; up to 0.50% molybdenum; up to 0.30% nickel; up to 0.10% niobium; up to 0.08% vanadium; up to 0.50% copper; up to 0.030% nitrogen; and up to 0.005% boron can be added. Addition of these fractions may, for example, enhance certain mechanical properties for a specified wall thickness of the product.
Other features and advantages of the present invention will be more readily apparent upon reading the following description of preferred exemplified embodiments of the invention.